In either case, the first steps to resolve this problem are:yum clean allyum remove epel-releaseyum install epel-release --enablerepo=extras

After this is done, you may need to do some manual work to uninstall and reinstall the EPEL packages that do not match your operating system version. For that, rpm -qa | grep el6 may help if you are running CentOS 7. This command will list the (EP)EL6 packages you have installed.

OK, that looks like CentOS 7 then. See my above message for further instructions. After you have cleaned up your system of EL6 packages you should yum update to upgrade to 7.5.1804.

As for remi-release, you will need to yum remove remi-release, and then install the el7 equivalent if you need packages from that repository. Remi's pages have instructions for installing the correct remi-release.